Project Summary/Abstract Acute leukemias are the most common cancers in children and infants. In particular, leukemias caused by translocations in the MLL gene are associated with poor prognosis. In order to improve treatment options a deeper understanding of the molecular players in MLL cancers will be required. Our lab has discovered that Prdm3 is required for leukemic transformation by the MLL translocation oncoprotein MLL-AF9. Prdm3 is a dual methyltransferase and transcription factor; although Prdm3 contains a lysine methyltransferase domain, it does not appear to act on histones. Therefore, we are interested in how Prdm3 methyltransferase activity contributes to leukemogenesis. To address this question, biochemical studies will be performed to further characterize the methyltransferase domain. Prdm3 will also be assayed for potential automethylation towards a ?histone mimic? motif at its N- terminus. This motif contains lysine and arginine residues at positions strikingly similar to those in the histone H3 tail. This raises the strong possibility that the histone mimic motif may be automethylated by Prdm3, or methylated by another cellular methyltransferase. Many previously reported protein interactions involving MLL and Prdm3 include chromatin modifying enzymes and transcription factors, suggesting that crosstalk between epigenetic modulators is key in the pathogenesis of leukemia. Finally, I will tie the biochemical activity of this protein back to leukemia in vivo through addback of Prdm3 methyltransferase domain mutants in a murine transplantation assay. This study will provide insight into the molecular events that drive leukemogenesis, and will guide further efforts towards developing effective cancer therapies.